Recently I got a question for a philosophy class on how the following set of statements is consistent: "All dragons love Katy Perry. All dragons hate Katy Perry." And we have to think about how to explain that this is consistent. I'm struggling to understand how to make that explanation.

I don't want to do your

I don't want to do your assigned coursework for you, so I'll say just this: Think about how modern logic translates "All S is P" and "All S is not P" into symbols.

Is truth binary? Can there be degrees of truth? Must we say a statement is either true or false? In everyday conversations we do say things like “The story is only 30% true.”

I'd say yes, truth is binary.

I'd say yes, truth is binary: there are no degrees of truth in between 0 and 1. For two reasons, I think that every proposition has exactly one of two truth-values, true or else false . (1) I'm not aware of any problem in logic or philosophy whose solution really does require positing fewer or more than two truth-values. (2) To my knowledge, every system of logic or semantics that accommodates other than two truth-values has consequences that are deeply implausible. These systems include three-valued logic, infinite-valued logic, supervaluation semantics, and others. In light of (1), I see no reason to flirt with those implausible consequences. When solving a problem seems to require other than two truth-values, the real trouble lies somewhere else. Or so it seems to me. Now, to say that every proposition is true or false is not to say that every sentence is true or false, not even every grammatically correct declarative sentence. For instance, the self-referential Liar sentence "This...

This is a follow up to a question answered by Dr. Maitzen on December 31 2020. The statement really was “Only if A, then B”. It came up on a test question that asked the following: “If A, then B” and “Only if A, then B” are logically equivalent. True or false? The answer is ‘false’, apparently. I reasoned that “Only if A, then B” is maybe like saying “Necessarily: if A, then B”, and this is clearly different from saying simply “If A, then B”. But I’m not sure. Any chance you might be able to help me see why “If A, then B” and “Only if A, then B” aren’t equivalent? Clearly they say different things, but I’m just not sure how to put my finger on the difference. I really appreciate the help. Thank you again.

It sounds to me as though

It sounds to me as though your teacher may be using the awkward expression "Only if A, then B" as a way of asserting the biconditional "A if and only if B," which is equivalent to the biconditional "B if and only if A." As I say, the expression is awkward, but in any case I wouldn't read it as adding a modal operator like "Necessarily" to the conditional "If A, then B." Whoever wants to say "necessarily" really needs to use that word. Other than your teacher's decision, I can't think of any reason to treat "Only if A, then B" as the biconditional "A if and only if B." The form "Only if A, then B" isn't something you'll encounter in idiomatic English. Competent speakers wouldn't say, "Only if all humans are mortals, then all nonmortals are nonhuman." Instead, they'd say "All humans are mortals if and only if all nonmortals are nonhumans." But it's probably wise to follow your teacher's decision, at least until you're done with the course!

I have recently compared two philosophy texts which are very very close in material they present: A Concise Introduction to logic 12th edition by Patrick Hurley and Introduction to Logic by Irving Copi & Carl Cohen 12th edition. I have a question about the logical Equivalence Rule Material Implication which states where ever P imples Q appears one can substitute Not P or Q and vice versa. I noticed if Not P or Q is Implicated the NOT is always on the left hand side. There is no instance of Q or Not P and the rule Material Implication being applied. My question is if I am given "Q or Not P" can I apply Material Implication as written or must I commutate "Q or Not P" to get "Not P or Q" and then use the Material Implication rule? It seems all is done to avoid using material implication with a negative disjunct on the right hand side. What is the deal with that? In other words, Would I get false conclusions if I deduce Q or Not P as Not Q or Not P? I am correct in guessing this may be the case? I...

Using ">" for material

Using "> " for material implication, (P > Q) is equivalent to each of (~ P v Q) and (Q v ~ P). So you can deduce either of those disjunctions. I think it's just a matter of convention to favor the first of them. The reader is expected to be notice the equivalence of the two disjunctions. Now, (Q v ~ P) is certainly not equivalent to (~ Q v ~ P). From Q, you can infer the first of those disjunctions but not the second. The disjunction (Q v ~ P) is equivalent to (P > Q), whereas the disjunction (~ Q v ~ P) is equivalent to (P > ~ Q) and (Q > ~ P).

I'm confused about the nature of antecedents and conditionals like: (i) "Only if A, then B". I was told in my logic class that antecedents are always sufficient conditions and consequents are always necessary conditions. But if that's the case, then the antecedent in (i) "Only if A" is a sufficient condition. Particularly a sufficient condition for B. But saying "Only if A, then B" means that A is a necessary condition for B as well. So it appears that the antecedent in (i) is both a sufficient and necessary condition. But that doesn’t seem right, given that (i) is equivalent to (ii) If B, then A. And this means A is only a necessary but not a sufficient condition for B. Option 1: Maybe antecedents only are sufficient conditions in simple conditionals like (iii) “If A, then B”; but they aren’t sufficient conditions in conditionals like "Only if A, then B". That might be right. Option 2: On the other hand, we might say "Only if A" just seems to be an antecedent but isn't really. That would...

Like you, I'm puzzled by the

Like you, I'm puzzled by the form of the conditional "Only if A, then B." It doesn't seem to be idiomatic English. One might say "Only if you go to the party will I go," but one wouldn't say "Only if you go to the party, then I will go." That would be unidiomatic. So I presume that the conditional form you're learning is "Only if A, B" rather than "Only if A, then B." I would interpret "Only if A, B" as stating that A is a necessary condition for B, and therefore implying that B is a sufficient condition for A. If one wants to say that A is both necessary and sufficient for B, then one can say "If and only if A, B" -- although "A if and only if B" would be a smoother way of saying it. In any case, make sure that your logic teacher really did say "Only if A, then B" and, if so, ask if he/she meant to say "Only if A, B."

Let ‘B’= to be; let ‘~B’=not to be. P1: B v ~B P2: ~B C: ~B P2 is the negation of the left disjunct in P1, not the affirmation of the right disjunct in P1. P1: To be or not to be. P2: Not to be. C: Not to be. It seems to me that, argumentatively, there’s a difference between affirming ‘not to be’, the right disjunct, and negating ‘to be’, the left disjunct. It just happens that, in this case, what’s affirmed and what’s negated are logically equivalent. Is there a convention for conveying that argumentative difference? Also, can you recommend any articles or books where I can learn more about issues like this? Thank you very much :)

Interesting question! I think

Interesting question! I think you're right that there's something peculiar about this disjunctive syllogism: (1) B v ~ B (2) ~ B (3) ~ B You say that (2) must be the negation of (1)'s left disjunct rather than the assertion of (1)'s right disjunct, even though both of those are syntactically the same. You may find allies in those who distinguish between (i) denying or rejecting a proposition and (ii) asserting the proposition's negation. See Section 2.5 of this SEP entry . But here's a different diagnosis. Although (1)-(3) is a valid argument, and even a valid instance of disjunctive syllogism, the argument is informally defective because premise (1) is superfluous: (1) isn't needed for the argument's validity. Furthermore, anyone justified in asserting (2) is thereby justified in asserting (3) without need of (1). This argument is similar: (4) ~ B v B (5) ~ ~ B (6) B The claim that (5) is the negation of (4)'s left disjunct is at least as plausible as the claim that (2) is the negation of (1...

There is an infinite number of words - "ONE", "TWO", "THREE"... etc. Every word has a definition. Every definition consists of letters. There is a finite number of arrangement of letters; thus there is a finite number of definitions. Thus there is at least one word that doesn't have a definition. Paradox?

There is a finite number of

There is a finite number of arrangements of letters; thus there is a finite number of definitions. Is that true if we're allowed to use each letter an increasing number of times? If our stock of letter tokens increases without limit, then can't the number (and length) of our definitions also increase without limit? Certainly the names of the numbers will tend to get longer as the numbers they name increase, and those names will reuse letters to an ever-increasing degree.

Say I have a sequence of numbers - 1,2,3,4,5,6,7. I add 1 to 7 to create the next number in the sequence,8. The sequence is finite. I add 1 to 8 to get the next number in the sequence, 9. The sequence is finite. I keep on going... At what point does my sequence become infinite? How can my sequence ever become infinite?

I assume that there's some

I assume that there's some nonzero minimum time, however brief, that you require to perform each step of addition. In that case, you will never produce an infinite sequence of numbers: that is, there is no finite time at which you will have produced an infinite sequence of numbers. That fact doesn't imply that the positive integers aren't an infinite sequence of numbers -- only that you can't produce them in the described way in a finite amount of time.

Keep in mind I'm a complete novice in philosophy, especially when it comes to the literature. I might misrepresent some positions completely. Please call me out. In short: The determinist states: Our decisions are bound to causation, and thus we are not truly free. This statement implies that the only way for free will to exist would be to detach an agent from causation; as long as some factors affect out motivation to do something, we are not truly free. The determinist thus claims that the only way for a choice to be free is that there would be some force acting above the physical reality, especially when it comes to cognition and decisionmaking. Thus only in a dualistic reality is free will possible. I have a few problems with this: 1. This method of defining free will seems to consequentally destroy the agent. If we were to be able to decide what we want, we'd, at least apparently, fundamentally be nothing. How would it be possible to even assign a different "want" to ourselves without that want...

You wrote: "The determinist

You wrote, "The determinist states: Our decisions are bound to causation, and thus we are not truly free." In the context of free will, what you say describes not determinists in general but only hard determinists, i.e., those determinists who also say that determinism rules out free will. The other kind of determinists -- soft determinists -- accept determinism but say that it doesn't rule out free will and may indeed be essential to acting freely. Unlike hard determinists, soft determinists allow for the combination of determinism, free will, and moral responsibility. You'll find details in this SEP entry .

Hi, While reading aristotle and aquinas on part whole relationship i often read the phrase "something qua itself and qua something else" as in man qua headed or qua an animal, what do they mean by that ? and how can something be qua itself and at the same time be as something else ? Isnt that a contardiction ? Thanks in advance

In this context, it sounds as

In this context, it sounds as though "qua" is being used to mean "considered as." So, for example, qua sentient being (i.e., considered as a sentient being) you have particular rights, while qua adult citizen (i.e., considered as an adult citizen) you have those rights plus additional rights, such as the right to vote. I see no contradiction here.